In the list of the advantages of linear regulated supplies set out above, the one that seems to have most appeal to people is the first. It allows an amplifier to approximate more closely to a perfect voltage source, which would give exactly twice the power into 4 O than it gives into 8 O. In the not always rational world of hi-fi, this kind of amplifier behavior is often considered a mark of solid merit, implying that there are huge output stages and heavyweight power supplies that can gracefully handle any kind of loudspeaker demand. I disagree, for the reasons set out above, but let's follow the train of thought for a bit, until it derails.

A regulated supply clearly gives a closer approach to this ideal than an unregulated supply whose voltage will droop when driving the 4 O load. However, even if the regulated supply is as stiff as a girder of pure unbendium, there will still be load-dependent losses in the output stage that will make the 4 O output less than twice that into 8 O.

Assume for the moment that we have an amplifier which gives 100 W into 8 O. There will be emitter resistors in the output stage, and the lowest value they are likely to have is 0.1 O. (There are good reasons why these resistors should be as low as practicable, because this improves linearity as well as efficiency – see Chapter 6.) These resistors are in series with the output and so form a potential divider with the load. Their presence alone, without considering other losses such as increased output device Vbe values at higher currents, and the wiring resistance, will cause the 4 O output to be 195.1 W rather than 200 W. That perfect voltage source is not so easy to make after all.

However, to make a rather ambitious generalization (and all generalizations are of course dangerous) it can be said that the power deficit from this cause is rather less than that due to unregulated supply rails drooping, which can cause twice the loss in terms of watts. This factor depends very much on how big the mains transformer is, how big the reservoir capacitors are (because that affects the depth of the ripple troughs, which is where clipping occurs first) and so on – I said it was a generalization. It is therefore perhaps worthwhile to look a little closer at the regulated supply issue.

I was once faced with this situation: the managing director wanted exact power doubling in a high-power design, but I was less than enthusiastic about trying to make heavy-current regulated power supplies work dependably. Time for some thought.

If you accept that there is no problem in making a hum-free amplifier that runs from unregulated and ripply rails – which is emphatically true, as demonstrated in the second half of this chapter – then the function of the regulators is simply to keep part of the supply voltage away from the amplifiers. In effect, the output stage is a giant clipping circuit. So why not do the clipping at the input of the amplifier, where it can be done with a couple of diodes, and go back to an unregulated power supply? The idea is shown in Figure 9.1.

Figure 9.1: Putting a small-signal clipping circuit at the amplifier input to emulate a regulated power supply

The electrical power previously wasted in the regulators is now absorbed by the output devices, perhaps necessitating a bit more heat-sinking, but all the complications of regulators disappear. As with a regulated supply, the clipping will be clean and uncontaminated by ripple – in fact probably cleaner because a small-signal clipping circuit will have no time-constants that may gather unwanted charges during overload.

Now you may think that this is cheating – the managing director certainly did, but even he was forced to admit that what I proposed was functionally identical to an amplifier with regulated supplies, and much cheaper. However, the idea of deliberately restricting amplifier output – and this new approach simply makes it obvious that that is what regulated supplies do – did not appeal to him any more than it does to me, and the project went forward with unregulated supplies. And no hum.

In the foregoing argument there is one point that has been oversimplified a little. Making a small-signal clipping circuit is straightforward. Making a clipping circuit that is wholly distortion-free below the clipping point is anything but straightforward. As I described in Chapter 2, it can be done, with some non-obvious circuitry. You will, I hope, forgive me for not revealing it at the moment, but I rather hope that someone might buy the idea off me.

This is initially only going to be of benefit for the big boys again!! While the smaller organisations will have to wait for an out the box equivalent.
The only difference is it has a multi-volt flux capacitor, apologies for the poor attempt to a bad joke!!
Nic
kdweb.co.uk

Hi Bill
Regarding your comment:
"It's also worth noting that secondary (load) current does NOT increase flux density. Only the primary voltage affects flux density for a given design."
It's very refreshing to hear someone with strong links to the audio community expressing this simple but so frequently misunderstood truth. Time and again I wearily see reviewers talking with great approval of (needlessly) huge power transformers with high current ratings, used in order "to avoid saturation".
The problem doesn't seem to be confined to the "hi-fi" community either: one close friend who's a seasoned amplifier designer of some repute was surprised when I put him right on this point at a recent AES convention. The ignorance of transformer basics seems so widespread that I'd actually thought of submitting an AES tutorial paper called "The Misunderstood Transformer" (or something like that). I've held back to-date not least because although I've designed plenty of power transformers (for low-frequency and HF switched-mode use) I've less experience of detailed design for high quality audio, and there are other luminaries such as your good self (and dare I mention Brian Sowter, Per Lundahl...) who have greater knowledge here. Still seems like a good idea though.
Perhaps we could get some T-shirts made bearing the slogan "The Volts Determine The Flux" !! :-)
Kind regards
Mike Turner
http://www.aes.org/aes/mike-turner
http://uk.linkedin.com/pub/mike-turner/1/b8a/b7

Although much is made of the "low" external magnetic field of toroids, they must be rigorously constructed to live up to that reputation. First, each winding must COMPLETELY cover the entire core. In most commercial units, the magnetic field emanates from the egress of wire leads, where core coverage has a gap. Second, virtually all power transformers are designed to be as cheap as possible. Less core and less copper are used in designs that operate very near magnetic saturation of the core. But the downside is that, as saturation is approached, radiated magnetic field drastically increases. It's also worth noting that secondary (load) current does NOT increase flux density. Only the primary voltage affects flux density for a given design. Therefore, a design on the verge of saturation at 115 VAC will likely be a problem at 130 VAC. To make any power transformer magnetically-friendly, simply operate it at reduced primary voltage (with reduced secondary voltages, of course). Another problem with toroids, because they don't have even the smallest air gaps in the magnetic path, is inrush current when they're first turned on. In large transformers, this can cause nuisance breaker tripping. I think the advantages of toroids are usually over-stated. -- Bill Whitlock, president & chief engineer, Jensen Transformers, Inc. www.jensen-transformers.com

While I agree that using a iron-core, 50/60 Hz transformer makes a lot of sense for audio amplifier design in the range of 10 -100W of output, I think that customers wish for smaller box, along with new requirements for standby power efficiency will push the use of switching power supplies more and more over time. These switching power supplies don't suffer the poor power factor of a bridge-rectified iron transformer with big filtering capacitor, plus they allow designer to use tricks like variable power rails to save power when the volume control position is low. These can also suppply the control voltage along with the power rail.
The initial investment in designing those may pay off for big manufacturers. This is obviously more difficult for a small shop to design and certify a line-level switching power supply for a power amp so there is a definite opportunity here for companies to design specialised off-the shelf unit for sale to the smaller audio components designers that can't afford to design them.
Pierre Proulx